We have had some difficulty hitting out original gravity on higher gravity beers, and I'm hoping some of you more experienced folks can help.

Just a little background info: We have an all-grain brewing setup, with a 20 gal Megapot with false bottom and most of our batches are 15 gal. We mash at about 152 degrees for 90 minutes, sparge with water around 170 degrees, then do a 90-minute boil. With more average gravity beers, we can usually get pretty close to our target gravity (+/- 0.002). However, our attempts at beers with target gravities of >1.075 or so usually come up short. I acknowledge that the original gravity is just a goal, but our latest attempt at an Imperial Chocolate Stout was off by about 0.028.

One of the things that jumps out at me is that when sparging to collect our target volume for the boil, we left a good deal of sugar behind. Should we have sparged with more water, which would have increased our volume and required a longer boil? Would increasing the boil time affect the beer by causing more carmelization?

Can you give me an example of your efficiency and grain weight from one of your lower gravity batches and, also, the weight of grain you were using for the Imperial stout. I can run some numbers to see if it's possible to hit that gravity on your system with a reasonable amount of water.

Sparging with more water is a possibility, but it does have the potential to add more kettle caramelization. Another possibility is to use the first runnings to make a smaller volume of a high gravity beer and the second runnings to make a second, lower gravity beer.

We recently brewed 15 gal of a brown ale, which had a post mash (after sparging) gravity of 1.050 at a volume of 16.5 gal. We used 31 pounds of grain, so the efficiency was 26.6. After boiling, our target OG was 1.059; we hit 1.057.

As for our chocolate imperial stout, we only brewed a 5 gal batch. We measured a post mash gravity of 1.062 at a volume of 6 gal. We used 20.5 pounds of grain, so our efficiency is a low 18.1. After boiling, ou target OG was 1.102, but we only hit 1.074.

We mashed and boiled for 90 min.

I also plan on checking my thermometer to see if it needs to be recalibrated... That will be a task for tomorrow.

Are you batch sparging? I did my calculations for batch sparging because it's easier to calculate.

Did you really only boil-off 1 gallon during a 90 minute boil of your Imperial Stout? Boiling off a little more will improve your OG, of course, and should improve your efficiency a bit by forcing you to run more water through the grain. However,...

I calculate that you had probably had ~100% conversion of starch into sugar for your Brown Ale, and only lost efficiency to the wort that remains absorbed by the grain.

For your Imperial Stout, if you had achieved the same Conversion Efficiency, then lost the same amount to the grain, you should have had 6 gallons of 1.090 wort before the boil, giving you 1.108 after the boil. For your 1.062 pre-boil gravity you would have had to only had ~70% Conversion Efficiency, all else being equal.

I would suggest looking at factors that might have hurt conversion:
• First-most, look at the grain crush. Did you use the same grain mill with the same setting?
• Secondly, you probably had to mash pretty thickly. That can inhibit starch gelatinization and conversion. Raising the temperature of the mash up into the alpha amylase range of 158-162°F for a while toward the end of your mash can improve conversion, in some cases 10-20%.
• A longer mash cay also improve conversion.

I advise you use the chart here to monitor the gravity of the mash liquor prior to taking the first runnings for beers that might have difficulty converting. You can also get his Batch Sparge Simulator, which can be very useful for calculating theoretical yields from particular grain quantities and sparge volumes. I used that here, so you can use it to double-check my math.

Thank you for sending the links - the information will be very helpful!

We have basically been batch sparging; however, after reading up on it some more, we haven't let the sparge water sit in the grain long enough to really extract enough sugar. Hopefully, the next batch will work out a bit better - we're starting to put together a recipe for a double ESB (similar to Double Old Thumper).